1. Field of the Invention
The invention relates to an electrostatic transfer reproduction apparatus and more particularly, to a digitally regulated power supply for the corona charging unit.
As is well known in the prior art, in an electrostatic copying system a photoconductive surface first is moved past a corona discharge unit which is intended to apply a uniform electrostatic charge to the surface. After leaving the corona unit, the surface moves past an exposure system at which it is exposed to a light image of the original to cause the charge to leak off in exposed areas and to be retained in relatively dark areas of the original. Following the exposure step, the latent electrostatic image is developed by the application of toner particles thereto. In some systems of the art, the image may be formed on a sheet of photoconductive material and in other systems the image may be formed on another photoconductive surface such as that of a drum or a belt and be transferred to a sheet of plain paper or the like following development.
It will readily be appreciated that copy produced in an electrostatic copying system is a function of the charge applied to the photoconductive surface (photoconductor) by the corona. Not only should the charge be uniform over the image portion of the photoconductive surface, but also it should not vary from copy to copy of the original. Certain of the corona systems of the prior art use as a power source a 240 volt alternating current extra high tension transformer and rectifier system to provide the relatively high direct current potential required to operate the corona. Corona supply systems of this type have a number of shortcomings. Fluctuations in the supply voltage result in wide variations of the corona or ionic current which, in turn, result in variations in the charge applied to the photoconductive surface on successive operations thereof. Variations in the charge level of the photoconductor surface from a uniform optimum norm directly translates into non-uniform and/or poor quality copies. In addition, the systems of the prior art have slow response times (on and off times) and are heavy, expensive and bulky.
The terms, Xerography, Photoconductive, Photoconductive Insulator, Photoconductive Insulating Plate, Corona Discharge, Corona Discharge Unit, and Electrostatic Charge, are defined as follows:
Xerography is an imaging process in which a photoconductive insulating plate is uniformly charged (sensitized) in the dark and subsequently exposed to a light image of the object to be copied to form an electrostatic image on the plate. The electrostatic image is made visible (i.e. developed) by the application of electroscopic powder particles which are attracted to the image. In transfer xerography these particles are then transferred from the plate to a transfer material (e.g., paper) by applying an electrostatic charge to the transfer material. The powder image is subsequently fused to the transfer material. The term "electrophotography" is synonymous with xerography. PA0 Photoconductive - the property of a material whereby an increase in electrical conductivity is caused in response to light. PA0 Photoconductive insulator - a material which is, in the dark, electrically insulating but which, on exposure to light, becomes electrically conductive in those areas exposed. PA0 Photoconductive insulating plate - the plate used in a xerographic process on which the electrostatic charge image is formed. The plate may be flat, curved, the surface of a drum, a scroll, or take any other form. The terms xerographic plate, image bearing plate, image bearing member, image plate, photoconductor (pc), organic photoconductor (opc), photoreceptor, xerographic drum, drum, insulating layer and photoconductive printing surface as used in xerography all indicate the photoconductive insulating plate. PA0 Electrostatic image - a charge pattern on the photo-conductive insulating plate conforming to the illuminated object being copied. The terms "latent image" and "latent electrostatic image" as used in Xerography are synonymous with electrostatic image. PA0 Corona discharge - the effect produced by applying a high voltage to a fine wire or point which causes the air in the immediate vicinity of the wire or points to ionize. The ions thus produced will flow to a conductor of lower voltage. In modern Xerographic equipment a Corona discharge is employed to charge the photoconductive insulating plate, to effect transfer of the developed image from the photoconductive insulating plate to the transfer material and for other purposes. PA0 Corona discharge unit - a structure, also termed in the art a Corotron, wherein the corona discharge takes place. Numerous such structures, employing a plurality of generally parallel corona wires oriented generally perpendicularly to the direction of scanning movement of a photoconductive surface relative to the charging system in an electrophotographic copier, or the like, are known to the art. PA0 Electrostatic charge - as used in Xerography, the charge of static electricity applied to a photoconductive insulating plate or to the transfer material. An electrostatic charge is an electrical charge. When applied to the transfer sheet the electrostatic charge effects transfer and also causes the transfer material to adhere electrostatically to the surface of the photoconductive insulating plate.
Reference is made to U.S. Pat. No. 2,548,452, entitled "Corona Triode Voltage Regulator", granted Apr. 10, 1951 to C. M. Turner.
Reference is made to U.S. Pat. No. 3,062,956 entitled "Xerographic Charging Apparatus", granted Nov. 6, 1962 to J. J. Codichini.
Reference is made to U.S. Pat. No. 3,122,634 entitled "Controlled Charging in Xerographic Copying Apparatus" granted Feb. 25, 1964 to Paul F. King.
Reference is made to U.S. Pat. No. 3,489,895 entitled "Regulated Electrostatic Charging Apparatus" granted Jan. 13, 1970 to H. J. Hollberg.
Reference is made to U.S. Pat. No. 3,604,925 entitled "Apparatus for Controlling the Amount of Charge Applied to a Surface" granted Sept. 14, 1971 to C. Snelling.
Reference is made to U.S. Pat. No. 3,699,335 entitled "Apparatus for Charging a Recording Element with An Electrostatic Charge of a Desired Amplitude" granted Oct. 17, 1972 to E. C. Giaimo, Jr.
Reference is made to U.S. Pat. No. 3,770,927 entitled "Corona Charger Configuration" granted Nov. 6, 1973 to P. J. Hastwell.
Reference is made to U.S. Pat. No. 3,805,739 entitled "Controlling Multiple Voltage Levels for Electrostatic Printing" granted Apr. 23, 1974 to R. F. Feldeisen, et al.
Reference is made to U.S. Pat. No. 3,819,942 entitled "Regulated Power Supply for Corona Charging Unit" granted June 25, 1974 to P. J. Hastwell, et al.
Reference is made to the publication "Dark Voltage Control System" by L. M. Ernst, IBM Technical Disclosure Bulletin, Vol. 17, No. 5, Oct. 1974, pg. 1408.
Reference is made to U.S. Pat. No. 3,586,908 entitled "Automatic Potential Control System for Electrophotography Apparatus" granted June 22, 1971 to R. E. Vosteen.
Reference is made to U.S. Pat. No. 3,335,274 entitled "Xerographic Charging Apparatus with Means to Automatically Control the Potential Applied to the Corona Wire" granted Aug. 8, 1967 to J. J. Codichini, et al.
Reference is made to U.S. Pat. No. 2,965,756 entitled "Electrostatic Charging Apparatus" granted Dec. 20, 1960 to R. G. Vyverberg.
It is usual to charge the xerographic plate by means of a corona generating device which when supplied with potential above the corona threshold produces an emission of corona ions. Representative embodiments of corona generating devices are disclosed in Walkup U.S. Pat. No. 2,777,957 and in Vyverberg U.S. Pat. No. 2,836,725.
Reference is made to U.S. Pat. No. 3,076,092 entitled "Xerographic Charging Apparatus" granted Jan. 29, 1963 to G. R. Mott.
Reference is made to U.S. Pat. No. 3,578,970 entitled "Variable Width Corona Discharge Apparatus With Means to Shield or Vary a Predetermined Length of a Corona Discharge Wire" granted May 18, 1971 to D. F. Michaud.